It sounds like there are a whole slew of issues going on here, not just one single thing that could easily be addressed.

1. I think you and your father need to go talk to a counselor. Your stepmom isn't stealing him away, but he may be so caught up in the newness that he's missing some of his priorities.
2. She's a step-parent for a teenager, but doesn't realize she can't be a parent to you. She can support you, help you, but she can't parent you. That's your father and mother's job (see #1 about him losing his priorities). Some time, again with a counselor, might help her find her role correctly as well as put him in the position he needs to be in.

These are just my opinions of course and others may have some better advice than me.